For more than two decades, organic materials have been investigated for their potential usefulness to non-linear optics (NLO). Most research has been focused on non-resonant non-linear effects, since the losses are smaller and the response times are shorter than at resonance. Resonant NLO effects, however, are interesting because they can be induced at very small optical powers. Resonant NLO effects also clarify the nature of optical transitions and dynamic processes which are not yet completely understood for organic materials. In previous work, the linear dielectric properties of the crystalline organic compound, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) were reported;
see, e.g., D. Y. Zang et. al., Applied Physics Letters, Vol. 58, pp. 562-564 (1991). However, no additional work on this compound has been reported as to any non-linear properties it may have.
Non-linear optical properties permit the fabrication of optical devices, such as optical modulators, switches, frequency doublers, etc. A desirable property of such modulators is a large resonant non-linear coefficient, n.sub.2, which determines how sensitive the modulator is to the intensity of the incoming radiation, and hence how easily the modulator is switched. In this connection, the non-linear index of refraction is given by the product of n.sub.2 and the intensity.